The dreaded "diff" CLUNK for Z cars by JMortensen

7 minute read

The dreaded "diff" CLUNK for Z cars

Compliments of JMortensen @ www.hybridz.org

Z cars are prone to clunking noises when they transition on and off power. There are many causes for the clunk, but the most common suspect that is almost never guilty is the differential itself. Time and time again we read about someone who "checks the backlash" in the differential by rocking the driveshaft back and forth, or jacks up one wheel and rocks it back and forth and thinks that they've found the source of the noise. Occasionally one of these people will take the diff out and wonder why they can't reproduce the clunk with the diff out of the car, or how it is possible that the backlash is in spec. Some more unlucky people go to the trouble of replacing the whole differential with a known good unit or worse, replacing all of the bearings in the differential, which is an expensive job, only to find out that the clunk is still there when they're all done.

Checking backlash with the diff installed is very very difficult; I would say impossible. If you grab the driveshaft and turn it back and forth, you're probably not going to be able to isolate the feel the backlash because of everything else moving, and you probably can't move the pinion enough to isolate the backlash without everything else also moving. First you have to overcome the resistance in the transmission, and by the time you do that and the shaft starts to move, you've already put too much force on the pinion to just move the pinion alone without also moving the ring gear. So when you move the driveshaft, what you're likely doing is moving the transmission gears until they lock solid, the driveshaft splines and u-joints, the pinion flange and pinion splines, the ring and pinion, the pinion gears and the cross pin shaft, the diff output stubs and the side gear splines, the CV joints or u-joints on the halfshafts, the stub axle and companion flange splines, wheel bearings, and any brake slack that might get taken up. It's not a good test of backlash, and is probably only good for finding a very big problem somewhere such as a u-joint that is about to fall out.

Likewise on the jacking one side of the car up and twisting the raised wheel back and forth. You're going to feel any and all slack from the wheel on one side to the wheel on the other. Slack could be coming from stub axle/companion flange splines for both sides, wheel bearings for both sides, CV or u-joints for both sides, output stub to side gear splines for both sides, and spider gears (and there is more slack in the spiders if it's an LSD). Some of this slack or gear lash is absolutely necessary. The transmission gears, ring and pinion, and spider gears in the diff MUST have lash, otherwise they'll burn up.

Even on a bench with the cover off and rocking the ring gear to check backlash you need to be a little careful. The 87 FSM says backlash on the R200 is .0051" to .0071", and getting the carrier to rock back and forth such a small amount without turning the pinion requires a bit of attention.

There are cases where diff backlash is a problem. There are many more cases (common enough drag racing issue that it shows up on the forums from time to time) where the cross pin shaft on the carrier wallows out its hole, and this leads to a larger amount of pinion rotation before the outputs begin to move.

More likely though, the ring and pinion is not the cause of your clunking. So what is? Well, it could be any number of things, but we can make a list of the most common issues pretty easily.

1. Diff strap/front diff mount. It’s hard to imagine that Nissan sold a million Z cars with this kluged design, but they did. The diff mount is below the front of the differential housing and the nose of the diff wants to lift when you apply power to it. The mount is rubber and tears pretty easily, so Nissan added a strap over the top of the diff to limit the movement. This is a less than satisfactory fix in practice, and over time or with added power the straps break and the mounts tear, and the nose of the diff can lift surprisingly far up. It can raise up so high that the driveshaft hits the transmission tunnel. Solutions: Ron Tyler’s diff mount (which he designed but other people are now producing), clamshell design diff mount addition where a bottom half is added to the diff mount which curves under the diff crossmember and prevents the mount from moving up, solid diff mounts also work but are generally not recommended because they put a lot of noise into the cabin and can cause stress at the crossmember causing it to break, and beefier replacement straps (usually metal or chain).

2. Mustache bar bushings. The diff hangs in the rear on the mustache bar. The mustache bar is insulated from the frame by rubber bushings, and they can wear out. These are typically replaced with polyurethane, which might solve a clunk but also transfers more diff noise into the car.

3. U-Joints. Worn driveshaft or halfshaft u-joints will definitely clunk on accel/decel. Replace as necessary. Just as checking backlash is difficult with the diff installed, I find checking u-joints on installed shafts damn near impossible. I have a friend who swears he can do it, but I always take the shaft out.

4. Control arm bushings. If the control arm bushings are worn out, they can allow movement which can cause a clunk. They are a royal pain in the ass to replace. Search: “spindle pin” for more details.

5. Loose bolts. A lot of these clunks are caused by the two big nuts that hold the diff to the mustache bar coming loose, or loose driveshaft bolts. Also check diff cover bolts and suspension mounting bolts.

If you have a clunk and you’ve gotten to this point and haven’t found the source, you’re into the weird stuff. At this point you need to work your way through the whole driveline looking for problems. Look for loose splines on drive shafts and axles, bad driveline mounts, anything. Arne over at www.classiczcars.com even reported that his clunk turned out to be caused by a woodruff key inside the transmission. “Mine was in the transmission itself. The woodruff keys that locate the drive gear on the front of the countershaft were worn, allowing the the gear to move on the countershaft. Clunk once under forward torque, and thud back under the reverse. Sounded for all the world like the clunk was coming from much farther back.”

Dealing with a clunk is like pulling the spindle pins on the rear control arms. It's a not so glamourous rite of passage that most of us go through. I'm hoping others will add their oddball clunk causes as well.